[Qgis-developer] [Server] performance questions

Fri Apr 22 08:20:21 PDT 2016

Hi,

I don't know the server code very well, so I may be wrong, but I think 
that making the server multithreaded rendering (MTR) capable should be 
possible. And while it is certainly often possible to increase the 
performance with filtering, rules and caching there are certainly also 
situations where MTR can help a server to generate quick responses (many 
layers, few requests).

As far as I can see, the main task would be porting the code from 
QgsMapRenderer to QgsMapRendererJob and QgsMapSettings. A first step 
into this direction can be found in a recently closed pull request [1], 
in particular this commit [2].

Concerning parallel DB connection, there's no difference between server 
and desktop use. This code is already in production use and should be 
fairly robust already.

I hope I didn't base this message on wrong assumptions and hope someone 
of the server developers can shed more light onto this.

Regards
Matthias

[1] https://github.com/qgis/QGIS/pull/2989/files
[2] 
https://github.com/qgis/QGIS/pull/2989/commits/4cba6393544aeb23569d6c15f19e67f4ff8bf19c

On 22/04/16 17:00, Andrea Peri wrote:
>
> I guess a more time consuming parameter is the number of vertex in the 
> feature involved in the map rather than the layer numbers.
>
> A.
>
> Il 22 apr 2016 3:27 PM, "Jörg Habenicht" <jh at mwerk.net 
> <mailto:jh at mwerk.net>> ha scritto:
>
>     Hi Andreas, dear list,
>
>     sorry for the warm up of this old topic.
>
>     There is in deed a need to enable multi threading in Qgis-Server.
>
>     In our company we use around 60 different layer to lay some topics on
>     the map. This may be an extreme example to read, but quite common
>     in our
>     environment, i.e. we can not lower this number significantly.
>
>     A warmed up QGis-Server process which already read and cached the
>     Configuration file qgis-xy.conf still needs around 20 seconds to
>     calculate the complete resulting map picture.
>
>     During this calculation there is only one thread working, serializing
>     the DB-requests, calculation the layer picture and mapping the picture
>     on the resulting picture.
>     The process is running on a multi core server. If we can utilize the
>     idle cores to do useful work, this can be an easy example to speed up
>     the process (significant).
>
>     I'd like to share my developing experience to do this change, but I
>     would need further information and some support to do some useful
>     patches.
>
>     I have scanned the git-Qgis sources (commit
>     81744ecf90a8f6c1e2e94fdacb07e3dca6987dcc) for a suitable
>     multithreading
>     entry. Found QgsMapRenderer::render() because of the line "while (
>     li.hasPrevious() )". It seems in this loop the layers are painted in
>     serial onto the resulting picture.
>     If I can build an auto parallel computing with QtConcurrent in this
>     place there seems to be the most gain. I mean to parallelize the loop
>     with concurrent DB requests, concurrent painting in separate pictures
>     and in serial map the single pictures onto the result picture.
>
>     Right now I don't know if I can do DB requests in parallel, or
>     where the
>     code for database requests is buried.
>     And how about the calls inside the loop, are there side effects
>     preventing multi threading?
>
>
>     Do you got hints, flames or suggestions?
>
>
>     cu
>     Jörg
>
>
>     Am 23.02.2016 um 16:39 schrieb Neumann, Andreas:
>     > Hi Régis,
>     >
>     > QGIS server is single-threaded. You can't simply turn on
>     multi-threaded
>     > rendering in QGIS server. The other issue is that each Apache FCGI
>     > process has it's own set of cache. So if you have 5-10 parallel
>     Apache
>     > threads or processes each thread/process has it's own cache. If your
>     > client hits a thread that has the cache unititialized you will
>     have wait
>     > longer until this cache is fillled.
>     >
>     > I once asked Marco whether one could
>     >
>     > a) have a shared cache for all Apache threads or processes
>
>     There is no shared cache, because Apache spawns QGis-Processes
>     with its
>     own memory layout. these Processes know nothing about its
>     neighbor, and
>     you have a hard time to exchange data between all processes.
>     How about a scheduler, which assigns requests to warmed up
>     Qgis-processes?
>
>     >
>     > b) support multi-threaded rendering
>     >
>     > But he said it would be very complicated to implement a) and b),
>     if not
>     > impossible. Maybe other devs have other views. Note that was about 2
>     > years ago, when I last discussed this with Marco.
>     >
>     > ---------------------
>     >
>     > Nevertheless I believe there would be room for performance
>     improvements
>     > in QGIS server - if interested parties share their efforts (be
>     it dev
>     > time or financial resources).
>     >
>     [snip]
>     >
>     > Andreas
>     >
>     [snip]
>     >
>     >
>     >
>     >
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>     --
>
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